I’m going to go with the “get Mordred” means “Collect Mordred”.

As in, I feel like Wiglaf is going to enter Mordred’s bedroom, possibly with the desire to wake him up or drag him out of bed forcefully. But, upon seeing Mordred, Mordred’s ability to get away with anything activates, and fight’s Wiglaf’s ability to do anything perfectly. They come to a stalemate, and Wiglaf picks up Mordred, still in his bedding’s, if not Mordred’s entire mattress, and carries him off for a day trip.

…possibly starting with a trip to the shops so that Mordred could buy something and not be in his pajamas all day.